For the choreography: prepare an outline of what you want to say, write down what information you want to convey, maybe use index cards to help your memory. Know what the audience should be able to remember after the talk, and emphasize these. If you introduce new concepts, explain them using examples that you think everybody is familiar with (or ask first if everybody is familiar with them). Prepare more than one way of explaining difficult material. Think of pictures or diagram you could use to explain concepts or relations between concepts.
For the rehearsal: you might think that
rehearsing a talk is a waste of time and that you prefer to be spontaneous.
It is in fact true that some people have the talent to give talks and speeches
without having to prepare them at all. But, be aware, most of us do not
have that gift And then, why should giving a talk be different than driving
a car, dancing, playing football, writing essays, playing guitar etc.?
To become proficient or even an expert in all of these activities you have
to practice, practice, and again practice. Thus, after you have prepared
your talk, practice it. First, without any audience. This allows you to
try out certain phrases, but also to get a feeling for the timing. Bad
time management is a clear sign of lack of preparation. Later, rehearse
before your friends. Ask them for their feedback-the more the better. Use
the feedback to improve future performances.